1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device using a liquid crystal having a ferroelectric phase and/or an antiferroelectric phase.
This invention also relates to an LCD device that uses a liquid crystal having a ferroelectric phase and/or an antiferroelectric phase, which can provide a stable alignment state with less alignment defects.
2. Description of the Related Art
It has been proposed to use a ferroelectric liquid crystal whose chiral smectic phase has a helical pitch smaller than the gap between substrates of a display device as a ferroelectric LCD device which can ensure gradation display. Ferroelectric liquid crystals of this type are classified into an SBF liquid crystal which has a memory property and a DHF liquid crystal which has no memory property (see "LIQUID CRYSTALS," 1989, Vol. 5, No. 4, pages 1171 to 1177).
The DHF liquid crystal is sealed between substrates, with the helical structure remaining intact. When a voltage whose absolute value is sufficiently large is applied between electrodes facing each other with a liquid crystal layer in between, the DHF liquid crystal becomes either a first alignment state (first ferroelectric phase) in which the average direction of the directors of the liquid crystal (LC) molecules is aligned to a first aligning direction or a second alignment state (second ferroelectric phase) in which the average direction of the directors of the LC molecules is aligned to a second aligning direction, in accordance with the polarity of the applied voltage. When the absolute value of the applied voltage is lower than the one which sets the DHF liquid crystal to the first alignment state or the second alignment state, the DHF liquid crystal becomes an intermediate alignment state in which the average direction of the directors of the LC molecules comes between the first and second aligning directions, due to the helical deformation of the molecule alignment. The intermediate gradation can be displayed using this intermediate alignment state.
In an LCD device using an SBF liquid crystal, the ratio of the area which has the LC molecules in the first alignment state to the area which has the LC molecules in the second alignment state varies in accordance with the applied voltage. The intermediate gradation can be displayed using a variation in this ratio.
A recent development is the use of the antiferroelectric liquid crystal. The antiferroelectric liquid crystal has three stable states of two ferroelectric phases and one ferroelectric phase, and is also aligned in the intermediate alignment state by the precursor or the like. The intermediate gradation can be displayed using this intermediate alignment state.
According to an LCD device which uses a liquid crystal having a ferroelectric phase and/or an antiferroelectric phase, the angle (tilt angle) .THETA..sub.1 of the LC molecules with respect to the direction of the normal line of a smectic layer when a predetermined positive voltage shown in FIG. 1A is applied should desirably be directed in the opposite direction to the direction of the tilt angle .THETA..sub.2 of the LC molecules when a predetermined negative voltage shown in FIG. 1B is applied, and have the same value as that of the tilt angle .THETA..sub.2. In other words, it is desirable that the applied voltage vs. transmittance characteristics for the application of voltages of the opposite polarities should be equal to each other.
Actually, the tilt angles .THETA..sub.1 and .THETA..sub.2 of the ferroelectric liquid crystal when the same voltages of the opposite polarities are applied differ from each other. The transmittance of the LCD device therefore varies in accordance with polarity of the applied voltage, resulting in irregular images in accordance with the display gradation or unstable display.
An LCD device which uses a liquid crystal having a ferroelectric phase uses an aligning film with a high pretilt angle to prevent the liquid crystal from having the chevron structure. To stabilize the alignment, a relatively thick aligning film is used.
The aligning film with a high pretilt angle is apt to cause an alignment defect, degrading the display quality. In addition, the thickness reduces the voltage to be applied to the liquid crystal and increases the drive voltage, thus increasing the consumed power.
Further, the process for bonding two substrates at the time of forming an LCD device bonds both substrates by an unhardened seal member which is baked at a high temperature of about 200.degree. C. This high-temperature treatment degrades the characteristics of the previously formed aligning film, which may cause an alignment defect on the completed LCD device. A similar problem occurs at the time of sealing the LC injecting inlet, formed in the seal member, by a thermosetting resin.
In an LCD device which uses a liquid crystal having a smectic phase such as a ferroelectric liquid crystal and an antiferroelectric liquid crystal, the liquid crystal is located between the substrates with a layer structure and is aligned in such a way that the normal direction of the layer is directed in accordance with the direction of an aligning treatment. When the characteristics of the aligning film is degraded by the heat treatment in the manufacturing process as mentioned above, however, the alignment itself becomes difficult. Further, a defect in one layer spreads to another layer. The heat-originated deterioration of the aligning film therefore is prominent in LCD devices which use a liquid crystal having a smectic phase such as a ferroelectric liquid crystal and an antiferroelectric liquid crystal.